pty in blocking mode with buf <= rx < min

test.c

// compilation: gcc -D_GNU_SOURCE -O0 -g -ggdb test.c
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <termios.h>
#include <unistd.h>
#include <assert.h>

#define TEST_VMIN     5
#define TEST_RECEIVED 4
#define TEST_BUF      3

void set_ncanon(int fd, int min, int time);
void get_pty_pair(int* ptm, int* pts);

void main(void)
{
  int e, pts, ptm;

  get_pty_pair(&ptm, &pts);

  set_ncanon(ptm, 1, 0);
  set_ncanon(pts, TEST_VMIN, 0);

  e = write(ptm, "anything", TEST_RECEIVED);
  assert(e == TEST_RECEIVED);

  char buf[TEST_BUF];
  int res = read(pts, buf, sizeof (buf));
  assert(e > 0);

  printf("res=%d buf=%.*s\n", res, TEST_RECEIVED, buf);
}

// set fd in noncanonical mode by modifying the current
// termios settings of fd.
void set_ncanon(int fd, int min, int time)
{
  struct termios tio;

  tcgetattr(fd, &tio);

	tio.c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP | INLCR | IGNCR | ICRNL
                   | IXON);
  tio.c_oflag &= ~OPOST;
  tio.c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
  tio.c_cflag &= ~(CSIZE | PARENB);
  tio.c_cflag |= CS8;
  tio.c_cc[VMIN] = min;
  tio.c_cc[VTIME] = time;

  tcflush(fd, TCIFLUSH);
  tcsetattr(fd, TCSANOW, &tio);
}

// another way to set fd in noncanonical mode by overwritting current
// termios settings
// void set_ncanon(int fd, int min, int time)
// {
//   struct termios tio;
//
//   bzero(&tio, sizeof(tio));
//
//   tio.c_cflag = B38400 | CRTSCTS | CS8 | CLOCAL | CREAD;
//   tio.c_iflag = IGNPAR;
//   tio.c_oflag = 0;
//   tio.c_lflag = 0;
//   tio.c_cc[VTIME] = time;
//   tio.c_cc[VMIN] = min;
//
//   tcflush(fd, TCIFLUSH);
//   tcsetattr(fd, TCSANOW, &tio);
// }

void get_pty_pair(int* ptm, int* pts)
{
  int e;

  *ptm = open("/dev/pts/ptmx", O_RDWR | O_NOCTTY | O_SYNC);
  assert(*ptm > 0);

  e = grantpt(*ptm);
  assert(e == 0);
  e = unlockpt(*ptm);
  assert(e == 0);
  char* pts_node = ptsname(*ptm);
  assert(pts_node != NULL);

  *pts = open(pts_node, O_RDWR | O_NOCTTY | O_SYNC);
  assert(*pts > 0);
}

// another way to set fd in noncanonical mode by overwritting current
// termios settings
// void set_ncanon(int fd, int min, int time)
// {
// struct termios tio;
//
// bzero(&tio, sizeof(tio));

Never do this.

SUSv4 says this:

Since the termios structure may include additional members, and the standard members may include both standard and non-standard modes, the structure should never be initialized directly by the application as this may cause the terminal to behave in a non-conforming manner.

//
// tio.c_cflag = B38400 | CRTSCTS | CS8 | CLOCAL | CREAD;
// tio.c_iflag = IGNPAR;
// tio.c_oflag = 0;
// tio.c_lflag = 0;
// tio.c_cc[VTIME] = time;
// tio.c_cc[VMIN] = min;
//
// tcflush(fd, TCIFLUSH);
// tcsetattr(fd, TCSANOW, &tio);
// }